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m/^(?! $a ) .*?  $end/sx

m/^(?! .*? $a ) .*? $end/sx

$_ = '...</code> ... foo';

...
    print "Match\n";
}
# no match

m/^ (?: . (?! $a ) )* $end /sx

$_ = 'foo...</code> ... ';

...
    print "Match\n";
}
# match

m/^ (?: (?! $a ) . )* $end /sx